Methods of adjusting glass melting and forming temperatures without substantially changing bending and annealing temperatures and glass articles produced thereby

ABSTRACT

A method is provided for adjusting, e.g., lowering, the melting and/or forming temperatures of a glass composition without substantially changing the bending and annealing temperatures of the glass composition. The method includes increasing the amount of CaO and decreasing the amount of MgO in the glass composition by the same or about the same amount.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to glass compositions having improvedmelting and refining characteristics and, more particularly, to methodsof adjusting a glass composition to lower the temperature(s) of themelting and/or forming viscosities without substantially changing thetemperature(s) of the bending and/or annealing viscosities of the glass.The invention also relates to glass articles made from the glasscompositions.

2. Technical Considerations

Glass manufacturers melt glass batch materials and refine the moltenglass to form glass articles. For example, in a conventional float glassprocess, glass batch materials are heated in a furnace or melter to forma glass melt. The glass melt is poured onto a bath of molten tin, wherethe glass melt is formed and continuously cooled to form a float glassribbon. The float glass ribbon is cooled and cut to form solid glassarticles, such as flat glass sheets. The particular batch materials usedand their relative amounts are selected based on the desired propertiesof the glass articles. Exemplary glass batch compositions are disclosedin U.S. Pat. Nos. 5,071,796; 5,837,629; 5,688,727; 5,545,596; 5,780,372;5,352,640; and 5,807,417, just to name a few.

As will be appreciated by one of ordinary skill in the glassmanufacturing art, glass composition properties can be defined based ontheir temperature and viscosity characteristics. For example, the“melting temperature” of a glass is conventionally defined as thetemperature at which the glass has a viscosity of 100 poises, which isconventionally referred to as the temperature of the “log 2”viscosity(i.e., the logarithm of the viscosity of the glass in poises is 2).Similarly, the “forming temperature” (log 4 viscosity), “bendingtemperature” (log 7.6 viscosity), “annealing temperature (log 13viscosity), and “strain point” (log 14.5 viscosity), are conventionallydefined as the temperatures at which the logarithms of the glassviscosity in poises are 4, 7.6, 13, and 14.5, respectively. The“liquidus temperature” is that temperature at which the glass begins todevitrify, which can cause undesirable haziness in the glass product.The difference between the forming temperature and the liquidustemperature is known as the “working range”. It is generally desirableto have a working range spanning more than 40° F. (22° C.).

Glass fabricators purchase flat glass sheets from glass manufacturersand process these glass sheets into various commercial products, such asarchitectural windows, mirrors, shower doors, automotive windows,insulating glass units, etc. Typically, this processing includes heatingthe flat glass sheets to bend the sheets and then controllably cool thesheets to anneal, temper, or heat strengthen the sheets. The bending,tempering and/or annealing temperatures for a particular type of glassare important economic factors in the fabrication process and cannot beeasily changed without substantially altering the existing fabricationprocess, which would be expensive and time consuming.

Due to increased tonnage and quality demand for flat glass products,flat glass manufacturers are under pressure to increase their glassproduction while reducing the cost of manufacturing the glass. Manyglass manufacturers are operating their glass furnaces at higher andhigher throughput and temperatures to meet the increased demand forglass. However, this need to increase glass production has resulted inseveral problem areas. For example, the operating temperature of aconventional flat glass furnace is typically on the order of 2850° F.(1564° C.). As more glass batch material is processed through thefurnace, more fuel is required to melt the increased amounts of glassbatch materials in a shorter time period. This increased fuel usage addssignificantly to the production cost of the glass sheets or articles andresults in a decreased thermal efficiency for the melting operation.Further, running the melter at increased throughput and at elevatedtemperatures can also damage the melter refractories, such as by causingthermal and/or chemical damage to the silica crowns and breast walls,which can lead to premature failure or collapse of the meltersuperstructure and solid defects in the glass.

Therefore, it would be advantageous to provide glass manufacturers witha method of adjusting a glass composition (and thus the batch materialsfrom which it is made) to provide a lower melting point to decrease fuelusage and potential damage to the melter while maintaining substantiallythe same bending and annealing temperatures as the starting glasscomposition.

SUMMARY OF THE INVENTION

The present invention provides a method of adjusting, e.g., lowering,the melting and/or forming temperatures of a glass composition withoutsubstantially changing the bending and/or annealing temperatures of theglass. In one aspect of the invention directed to glass compositionscontaining calcium oxide (CaO) and magnesium oxide (MgO), it has beendiscovered that increasing the amount, e.g., weight percent, of CaO anddecreasing the MgO by substantially the same amount (weight percent)results in glass having lowered melting and forming temperatures withoutsubstantially changing the bending and annealing temperatures of theglass.

In another aspect of the invention, a method of lowering the melting andforming temperatures of a glass composition includes replacing at leastsome of the CaO and/or MgO of the glass composition with a metal oxidewhose metal ion has a lower field strength than Ca⁺⁺ and/or Mg⁺⁺, e.g.,Ba⁺⁺ or Sr⁺⁺.

A glass composition having advantageous properties for flat glassmanufacture is also provided. In one embodiment, the glass compositionhas a melting temperature in the range of about 2570° F. to about 2590°F. (1410° C. to about 1421° C.) and a forming temperature in the rangeof about 1850° F. to about 1894° F. (1010° C. to about 1034° C.). Theglass composition has a bending temperature in the range of about 1300°F. to about 1350° F. (704° C. to about 732° C.) and an annealingtemperature in the range of about 1016° F. to 1020° F. (547° C. to 549°C.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the normalized deviation of selected parametersversus weight percent CaO for an exemplary (computer modeled) glasscomposition of the invention;

FIG. 2 is a graph of the normalized deviation of selected parametersversus weight percent CaO for another exemplary (computer modeled) glasscomposition of the invention; and

FIG. 3 is a graph of thermal efficiency versus weight percent CaO usedfor an eight month time period in which a glass making furnace wasoperated in accordance to the features of the invention.

DESCRIPTION OF THE INVENTION

As used herein, all numbers expressing dimensions, physicalcharacteristics, processing parameters, quantities of ingredients,reaction conditions, and the like used in the specification and claimsare to be understood as being modified in all instances by the term“about”. Accordingly, unless indicated to the contrary, the numericalvalues set forth in the following specification and claims areapproximations that may vary depending upon the desired propertiessought to be obtained by the present invention. At the very least, andnot as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical value should atleast be construed in light of the number of reported significant digitsand by applying ordinary rounding techniques. Moreover, all rangesdisclosed herein are to be understood to encompass any and all subrangessubsumed therein. For example, a stated range of “1 to 10” should beconsidered to include any and all subranges between (and inclusive of)the minimum value of 1 and the maximum value of 10; that is, allsubranges beginning with a minimum value of 1 or more and ending with amaximum value of 10 or less, e.g., 5.5 to 10. Additionally, any numericreference to amounts, unless otherwise specified, is “by weight percent”based on the total weight of the glass composition. The total ironcontent of the glass compositions disclosed herein is expressed in termsof Fe₂O₃ in accordance with standard analytical practice, regardless ofthe form actually present. As used herein, the terms “solar control” and“solar control properties” mean properties which affect the solarproperties, e.g., visible, IR, or UV transmittance and/or reflectance,of the glass.

The present invention provides a method of adjusting a glass compositionto change, e.g., lower, the melting and/or forming temperatures of theglass composition without substantially changing the bending and/orannealing temperatures of the glass. The invention also provides glasscompositions having improved melting and forming characteristics whichare particularly well suited for a float glass process. An exemplarymethod of practicing the invention will first be discussed and thenexemplary glass compositions of the invention will be discussed.

Although the present invention can be practiced with any type of glass,the invention is particularly well suited for flat glass compositions,such as soda-lime-silica glass compositions, having silica as the majorconstituent along with other melting and refining aids. A basicsoda-lime-silica glass composition is formed from a batch having silica(sand), soda ash (a carbonate of soda), dolomite (a carbonate of calciumand magnesium), limestone (a carbonate of calcium), and oxidizingagents, such as nitrate or sulfate. The limestone and dolomite act asfluxes to aid in dissolution of the silica and to improve the durabilityof the glass product. As will be appreciated by one skilled in the art,the relative amounts of the batch components depend upon the desiredcomposition of the glass to be made.

Cullet may be added to the batch materials either before feeding thebatch materials into the melter or during melting. The cullet may beclear glass or may include conventional coloring agents. The cullet mayalso include iron in either the ferrous or ferric states, although theferrous state is desired for most solar control glass products.

Additional materials may also be added to the batch which affect thefinal properties of the glass, e.g., solar properties such as infrared(IR) or ultraviolet (UV) transmittance, reflectance, or opticalproperties, aesthetic properties, and the like. Such materials includeelements or compounds of titanium, selenium, cobalt, cerium, vanadium,molybdenum, chromium, nickel, manganese or copper. Generally, as theamounts of the colorants increase, the visible, IR and UV transmittanceof the resultant glass decrease.

The glass compositions of the invention may include small amounts ofother materials, for example melting and refining aids, tramp materialsor impurities, such as elements or compounds of sodium, potassium,calcium, magnesium, manganese, aluminum, sulfur, strontium, zirconium,chlorine, cobalt, nickel, selenium, chromium, molybdenum, barium,titanium, cerium, tin, zinc or iron.

In the practice of the invention, it has been found that for a glasscomposition, particularly a soda-lime-silica flat glass composition,increasing the CaO in the glass composition by a selected amount (weightpercent) while decreasing the MgO by the same selected amount (i.e. thesame weight percent change as the CaO) or substantially the same amount(e.g., up to ±5 weight percent, e.g., ±5 weight percent or less, e.g.,±4 weight percent or less, e.g., ±3 weight percent or less, e.g., ±1weight percent or less, preferably ± less than 1 weight percent of theselected amount) while maintaining a substantially constant total amountof CaO+MgO (e.g., maintaining the total amount within ±5 weight percentof the starting total amount, e.g., within ±3 weight percent, e.g.,within ±1 weight percent, preferably within ± less than 1 weightpercent) decreases the melting and forming temperatures of the glasswithout substantially changing the bending and annealing temperatures ofthe glass. It is believed this result is based at least partly on thefact that the atomic field strength (conventionally designated z/a²where “z” is the ion charge and “a” is the internuclear distance betweenthe cation and anion) for the calcium ion (0.33) is less than the fieldstrength of the magnesium ion (0.45). This lower calcium ion fieldstrength is believed to result in lower calcium covalent bond strengthas compared to magnesium covalent bond strength, thus requiring lessshear force to break the calcium covalent bonds which results in lowerglass viscosity in the melting and forming temperature range.

In one particular embodiment of the invention, it has been discoveredthat increasing the relative amount (weight percent based on totalweight of the glass composition) of CaO with respect to MgO in the glasscomposition while maintaining a total amount (weight percent based ontotal weight of the glass composition) of CaO+MgO in the range of 12 to15 weight percent, e.g., 12.1 to 15 weight percent, e.g., 12.5 to 13.0weight percent, e.g., 12.8 to 12.9 weight percent, results in glasshaving lower melting and forming temperatures than before thisadjustment, without substantially changing the bending and annealingtemperatures of the glass. As used herein, the phrases “withoutsubstantially changing the bending and annealing temperatures” or“substantially maintaining the bending and annealing temperatures” meanthat the bending and annealing temperatures of the glass preferably donot change more than about 1° F. to about 10° F. (0.5° C. to 5° C.),preferably not more than about 2° F. to about 5° F. (1° C. to 3° C.),more preferably less than about 5° F. (3° C.), still more preferably notmore than about 4° F. (2.5° C.), even more preferably less than about 3°F. (2° C.), and most preferably less than about 2° F. (1° C.).

In Examples 1-5 presented below, various exemplary glass compositionsare modeled to show the affect of varying the weight percent of CaO andMgO in accordance with the practice of the invention while maintainingthe other glass components substantially unchanged. As will beappreciated by one skilled in the art, in order to form these glasscompositions, the batch components, e.g., limestone and dolomite, areadjusted to yield a desired glass composition.

Based on this new understanding of glass behavior, glass articles can bemade having relatively higher CaO and lower MgO amounts (weightpercents) than previously practiced without adversely affecting thefabrication parameters, e.g., bending and/or annealing temperatures, ofthe glass.

An exemplary glass composition incorporating features of the inventionis characterized as follows:

Component Weight Percent SiO₂ 70-75 Na₂O 12-15 K₂O 0-2 CaO >9 MgO <4Al₂O₃ 0-2 SO₃ 0-1 Fe₂O₃ 0-2 SiO₂+ Al₂O₃ ≧70 Na₂O + K₂O 12-15 CaO + MgO  12-13.5 CaO/MgO 2-5

As will be appreciated by one skilled in the art, other conventionalcomponents or ingredients such as colorants, solar control materials,tramp materials, etc. as discussed above may also be present in theglass.

In the above exemplary composition, the CaO is preferably greater than 9weight percent and the MgO is preferably less than 4 weight percentbased on the total weight of the composition. For example, the CaO canbe greater than or equal to 10 weight percent, e.g., 10 to 10.5 weightpercent, e.g., 10.25±0.25 weight percent. The MgO can be less than orequal to 3 weight percent, e.g., 2 to 3 weight percent, e.g., 2.5±0.5weight percent. The total weight percent of CaO+MgO is preferably about12.8 to 12.9, e.g., 12.85±0.05. Additional exemplary glass compositionsinclude:

Component Composition 1 Composition 2 SiO₂ 72.53 72.89 Na₂O 13.79 13.9 K₂O  0.02 0 CaO ≧9.1    ≧10     MgO ≦4    ≦3    Al₂O₃  0.03  0.03 SO₃0.2 0.2 Fe₂O₃ 0.5 0.1 SiO₂ + Al₂O₃ 72.56 72.91-73.01 Na₂O + K₂O 13.8113.9  CaO + MgO 12.85 12.69-12.8

The additional exemplary glass compositions immediately above preferablyprovide a melting temperature of less than 2600° F. (1425° C.), e.g.,2500° F. to 2600° F. (1370° C. to 1425° C.), e.g., 2570° F. to 2590° F.(1410° C. to 1421° C.), and a forming temperature less than about 1900°F. (1037° C.), e.g., 1800° F. to 1900° F. (981° C. to 1037° C.), e.g.,1850° F. to 1894° F. (1010° C. to 1034° C.). The glass preferably has abending temperature of less than about 1400° F. (759° C.), e.g., 1300°F. to 1400° F. (704° C. to 759° C.), e.g., 1300° F. to 1350° F. (704° C.to 732° C.), and an annealing temperature of less than about 1050° F.(565° C.), e.g., 1010° F. to 1050° F. (543° C. to 565° C.), e.g., 1016°F. to 1020° F. (547° C. to 549° C.).

While the above exemplary glass compositions are presented to describethe general concept of the invention, it is to be understood that theinvention is not limited to these specific exemplary embodiments.

As will be appreciated from the above discussion and the followingExamples, the glass compositions of the invention provide improvedmelting and refining characteristics while maintaining substantially thesame fabricating characteristics. For example, the decreased meltingtemperatures provided by the glass compositions of the invention meanless fuel is required to initially melt the glass batch components.Additionally, the resultant glass article formed in accordance with theglass compositions of the invention also has a lower melting point thanwould occur without the practice of the invention. This means that whena glass article of the invention is used as cullet in the glass melter,less fuel is required to melt the cullet which further reduces the fuelrequirements. Moreover, the glass article can be used by glassfabricators using their existing bending and annealing apparatus andmethods without the need for the fabricators to change the fabricatingparameters, e.g., bending and annealing temperatures, used to fabricatea commercial glass product. Further, limestone (CaO source) is typicallyless expensive than dolomite (CaO and MgO source). Therefore, increasingthe amount of CaO and decreasing the amount of MgO in the glasscomposition means that more limestone and less dolomite are needed inthe batch, which reduces the cost of the glass batch.

In a further aspect of the invention, in addition to changing therelative amounts of the CaO and MgO in a glass composition as describedabove, one or more components of the glass, such as CaO and/or MgO, canbe replaced totally or in part by a material having a lower fieldstrength. For example, the CaO and/or MgO can be replaced in whole or inpart by a material, such as an oxide, containing Ba⁺⁺ or Sr⁺⁺, whichhave a lower field strength than Ca⁺⁺ or Mg⁺⁺.

The following examples are presented to demonstrate the principles ofthe invention. However, the invention is not limited to the specificexamples presented.

PREDICTIVE EXAMPLE 1

A database of flat glass compositions and their respective temperaturerelated properties was developed. The database was primarily based oncommercial flat glass compositions made by the float glass process. Thisdatabase was then statistically modeled using commercially available“Data Desk” and “SAS” statistical programs to develop algorithms for thevarious glass characteristics, such as melting temperature, formingtemperature, bending temperature, annealing temperature, liquidustemperature, and working range. The resulting algorithms were optimizedusing the “Solver” program which is available in the EXCEL™ menu fromMicrosoft Corporation.

Table 1 shows the results of this computer modeling for varying the CaOand MgO amounts for a hypothetical glass composition characterized asfollows:

Component Weight Percent SiO₂ 72.53 ± 0.1 Na₂O 13.79 ± 0.1 K₂O  0.02 ±01  Al₂O₃  0.03 ± .01 SO₃  0.2 ± .01 Fe₂O₃  0.5 ± .01 SiO₂ + Al₂O₃ 72.56± 0.1 Na₂O + K₂O 13.81 ± 0.1 CaO + MgO 12.85 ± .05

TABLE 1 Working Wt. Wt. Range % % Melting Forming Bending Annealing (log4- CaO MgO Temp. Temp. Temp. Temp. liquidus) 9.20 3.65 2594° F. 1868° F.1343° F. 1022° F. 62° F. (1423° C.) (1020° C.) (728° C.) (550° C.) (34°C.) 9.40 3.45 2589° F. 1866° F. 1344° F. 1022° F. 61° F. (1421° C.)(1019° C.) (729° C.) (550° C.) (34° C.) 9.50 3.35 2587° F. 1865° F.1344° F. 1023° F. 60° F. (1419° C.) (1018° C.) (729° C.) (551° C.) (33°C.) 9.59 3.25 2585° F. 1865° F. 1344° F. 1023° F. 60° F. (1418° C.)(1018° C.) (729° C.) (551° C.) (33° C.) 9.69 3.15 2584° F. 1864° F.1344° F. 1023° F. 60° F. (1418° C.) (1018° C.) (729° C.) (551° C.) (33°C.) 9.79 3.05 2581° F. 1863° F. 1344° F. 1024° F. 59° F. (1416° C.)(1017° C.) (729° C.) (551° C.) (33° C.) 9.99 2.85 2577° F. 1861° F.1344° F. 1025° F. 56° F. (1414° C.) (1016° C.) (729° C.) (552° C.) (31°C.) 10.20 2.64 2573° F. 1859° F. 1344° 1026° F. 53° F. (1412° C.) (1015°C.) (729° C.) (552° C.) (29° C.) 10.30 2.54 2571° F. 1859° F. 1344° F.1027° F. 51° F. (1411° C.) (1015° C.) (729° C.) (553° C.) (28° C.) 10.402.44 2569° F. 1858° F. 1344° F. 1027° F. 48° F. (1409° C.) (1014° C.)(729° C.) (553° C.) (27° C.) 10.50 2.34 2558° F. 1857° F. 1344° F. 1028°F. 46° F. (1403° C.) (1014° C.) (729° C.) (553° C.) (26° C.)

As shown in the computer modeling results of Table 1, as the weightpercent of CaO in the composition increases from 9.20 to 10.50 (with thetotal weight percent of CaO+MgO in the composition remaining at12.84-12.85), the melting temperature of the glass drops from 2594° F.(1423° C.) to 2558° F. (1403° C.) and the forming temperature drops from1868° F. (1020° C.) to 1857° F. (1014° C.). However, the bendingtemperature of the glass only changes from 1343° F. (728° C.) to 1344°F. (729° C.) and the annealing temperature of the glass changes from1022° F. (550° C.) to 1028° F. (553° C.).

As also shown in Table 1, as the weight percent of CaO in thecomposition increases the working range of the glass compositionnarrows. In order to prevent or minimize this decrease in the workingrange, the weight percent of Na₂O+K₂O in the glass can be increasedand/or the weight percent of SiO₂+Al₂O₃ in the glass decreased asdesired. It is anticipated that changes on the order of 0.05 to 0.1weight percent in these components in the 9.9 to 10.5 weight percent CaOrange would be effective to maintain a working range spanning more than50° F. (28° C.).

PREDICTIVE EXAMPLE 2

Another glass composition was computer modeled as described above. Themodeled glass was characterized as follows:

Component Weight Percent SiO₂ 72.89 ± 0.1 Na₂O  13.9 ± 0.1 K₂O 0 Al₂O₃  0.02 ± 0.01 SO₃  0.2 ± .01 Fe₂O₃  0.1 ± .01 SiO₂ + Al₂O₃ 72.91 ± 0.1Na₂O + K₂O  13.9 ± 0.1 CaO + MgO  12.8 ± .11

Table 2 shows the computer modeling results for varying CaO and MgO forthe above glass composition.

TABLE 2 Working Wt. Wt. Range % % Melting Forming Bending Annealing (log4- CaO MgO Temp. Temp. Temp. Temp. liquidus) 9.01 3.79 2595° F. 1867° F.1340° F. 1015° F. 65° F. (1424° C.) (1019° C.) (727° C.) (546° C.) (36°C.) 9.11 3.69 2594° F. 1867° F. 1340° F. 1015° F. 64° F. (1423° C.)(1019° C.) (727° C.) (546° C.) (36° C.) 9.20 3.59 2592° F. 1866° F.1340° F. 1015° F. 64° F. (1422° C.) (1019° C.) (727° C.) (546° C.) (36°C.) 9.40 3.39 2589° F. 1865° F. 1340° F. 1016° F. 61° F. (1421° C.)(1018° C.) (727° C.) (547° C.) (34° C.) 9.50 3.29 2588° F. 1864° F.1340° F. 1016° F. 60° F. (1420° C.) (1018° C.) (727° C.) (547° C.) (33°C.) 9.60 3.19 2586° F. 1864° F. 1340° F. 1017° F. 58° F. (1419° C.)(1018° C.) (727° C.) (547° C.) (32° C.) 9.70 3.09 2585° F. 1863° F.1340° F. 1017° F. 57° F. (1418° C.) (1017° C.) (727° C.) (547° C.) (32°C.) 9.80 2.99 2584° F. 1862° F. 1340° F. 1017° F. 55° F. (1418° C.)(1017° C.) (727° C.) (547° C.) (31° C.) 10.00 2.79 2581° F. 1861° F.1341° F. 1018° F. 51° F. (1416° C.) (1016° C.) (727° C.) (548° C.) (31°C.) 10.10 2.69 2579° F. 1861° F. 1341° F. 1019° F. 48° F. (1415° C.)(1016° C.) (727° C.) (548° C.) (27° C.) 10.20 2.59 2579° F. 1860° F.1341° F. 1020° F. 47° F. (1415° C.) (1016° C.) (727° C.) (549° C.) (27°C.) 10.30 2.49 2578° F. 1860° F. 1340° F. 1020° F. 46° F. (1414° C.)(1016° C.) (727° C.) (549° C.) (26° C.) 10.40 2.39 2577° F. 1859° F.1340° F. 1021° F. 46° F. (1414° C.) (1015° C.) (727° C.) (549° C.) (25°C.) 10.50 2.29 2576° F. 1859° F. 1340° F. 1021° F. 44° F. (1413° C.)(1015° C.) (727° C.) (549° C.) (29° C.) 10.60 2.19 2573° F. 1858° F.1341° F. 1022° F. 36° F. (1412° C.) (1014° C.) (727° C.) (550° C.) (20°C.)

As shown in Table 2, increasing the weight percent of CaO from 9.01 to10.60 in the glass while simultaneously decreasing the weight percent ofMgO by substantially the same amount decreases the melting temperatureof the glass about 22° F. (12° C.) and the forming temperature about 9°F. (5° C.) while the bending temperature of the glass changes only 1° F.(0.5° C.) and the annealing temperature changes only 7° F. (4° C.).

As discussed above, the weight percent of Na₂O+K₂O can be increasedand/or the weight percent Of SiO₂+Al₂O₃ decreased as desired to adjustthe working range to be greater than 50° F. (28° C.), if desired.

PREDICTIVE EXAMPLE 3

Another glass composition was computer modeled as described above. Themodeled glass composition had the following components:

Component Weight Percent SiO₂ 72.80 Na₂O 13.90 K₂O 0.03 Fe₂O₃ 0.10 CaO +MgO 12.74

The results of the computer modeling are shown graphically in FIG. 1,with the change in the parameters presented as normalized deviationswith reference to a baseline value (0 value). The “0 values” for thepresented parameters were:

melting temperature 2600° F. (1427° C.) forming temperature 1868° F.(1020° C.) bending temperature 1344° F. (729° C.) annealing temperature1013° F. (545° C.) working range  81° F. (45° C.) liquidus temperature1787° F. (975° C.) strain point  943° F. (506° C.)

As shown in FIG. 1, while the melting temperature of the glass dropssignificantly as the relative amount of CaO increases, the bending andannealing temperatures of the glass remain substantially unchanged.

PREDICTIVE EXAMPLE 4

A further glass composition was modeled having the followingcomposition:

Component Weight Percent SiO₂ 72.41 Na₂O 13.78 Al₂O₃  0.16 Fe₂O₃  0.48CaO + MgO 12.84

The results of the computer modeling are presented graphically in FIG. 2as normalized deviations from a 0 value in similar manner to FIG. 1. The“0 values” for the various parameters were:

melting temperature 2619° F. (1437° C.) forming temperature 1870° F.(1021° C.) bending temperature 1335° F. (724° C.) annealing temperature1015° F. (546° C.) working range  61° F. (34° C.) liquidus temperature1809° F. (987° C.) strain point  946° F. (508° C.)

EXAMPLE 5

In addition to the computer modeling described above, the invention wastested to determine the effect of the practice of the invention on thethermal efficiency of a conventional glass melter. As used herein, theterm “thermal efficiency” means the theoretical amount of fuel requiredto melt a given amount of glass batch materials (assuming 2.5 millionBTU to melt 1 ton of batch materials and 1.7 million BTU to melt 1 tonof cullet) divided by the actual amount of fuel used. The term “%thermal efficiency” is the thermal efficiency multiplied by 100. Theglass composition tested was characterized by:

Component Weight Percent SiO₂ 72.56 Na₂O + K₂O 13.85 Fe₂O₃  0.49 CaO +MgO 12.89

Batch materials to form this glass composition were melted in a glassfurnace and FIG. 3 shows the variation in thermal efficiency as theamounts of the batch materials were adjusted so that the relative amount(weight percent) of CaO in the glass composition was increased whilesimultaneously decreasing the MgO in the glass composition by the sameamount (weight percent). The thermal efficiency generally increased fromabout 32.5% to about 35% as the weight percent CaO increased from about9.0% to about 9.4%.

1. A glass composition, comprising: a. SiO₂ 70 to 75 weight percent b.Na₂O 12 to 15 weight percent c. K₂O 0 to 5 weight percent d. CaO>9weight percent e. MgO<4 weight percent f. Al₂O₃ 0 to less than 1.6weight percent g. SO₃ 0 to 1 weight percent h. Fe₂O₃ 0 to less than 0.65weight percent wherein SiO₂+Al₂O₃≧70 weight percent Na₂O+K₂O 12 to 15weight percent CaO+MgO 12 to less than 13.4 weight percent CaO/MgO 2.33to 5 wherein the glass composition has a log 2 viscosity in the range ofabout 2570° F. to about 2590° F. (1410° C. to 1421° C.) and a log 4viscosity in the range of about 1850° F. to about 1894° F. (1010° C. to1034° C.).
 2. The composition according to claim 1, wherein CaO is inthe range of greater than 9 to 12 weight percent.
 3. The compositionaccording to claim 1, wherein CaO is in the range of 9.1 to 11 weightpercent.
 4. The composition according to claim 1, wherein MgO is in therange of 2 to less than 4 weight percent.
 5. The composition accordingto claim 1, wherein CaO+MgO is in the range of 12.5 to less than 13weight percent.
 6. The composition according to claim 1, wherein theratio of CaO to MgO is 2.77 to
 5. 7. The composition according to claim1, wherein the glass composition has a log 7.6 viscosity in the range ofabout 1300° F. to about 1350° F. (704 to 732) and log 13 viscosity inthe range of about 1016° F. to about 1020° F. (547° C. to 449° C.). 8.The composition according to claim 1, wherein the melting point of theglass composition from the log 2 viscosity reduces fuel usage inpreparing the glass.
 9. The composition according to claim 7, whereinthe melting point of the glass composition from the log 2 viscosityreduces fuel usage in preparing the glass and the bending and annealingtemperatures of the glass from the log 7.6 viscosity in the range ofabout 1300° F. to about 1350° F. (704° C. to 732° C.) and a log 13viscosity in the range of about 1016° F. to about 1020° F. (547° C. to549° C.) are in the range for a higher melting glass.